1. Field of the Invention
The present invention relates to a coding apparatus which codes data to be transmitted.
2. Description of the Related Art
Recently, almost all documents used in offices are produced by personal computers or word processors. There are relatively few documents produced through handwriting.
Most documents produced by personal computers or word processors include letters and ruled lines, but typically do not include photographs.
In images of these documents, a certain degree of regularity is present in sizes, shapes, intervals, arrangements and so forth of letters. In contract to this, in images of photographs or handwritten documents, such characteristics are not present.
In an apparatus which treats an image as electronic information such as a facsimile machine or a digital copier, an input image is optically scanned through a scanner or the like so that the input image is converted into electronic information.
An amount of electronic information obtained through this conversion depends on the resolution and the size of the image. This amount becomes larger as the resolution or the size of the image increases. Ordinarily, the electronic information obtained through such conversion is stored in a storage area temporarily. Therefore, as the amount of information increases, the necessary capacity of the storage area increases.
Especially, recently, there is a trend that the resolution increases. Thereby, the necessary capacity of a memory or a disk for storing electronic information of images increases, and the costs therefor increase.
In order to solve this problem, there is a method for efficiently coding an input image. Typical examples of image coding are MH, MR and MMR methods which are used in facsimile machines and so forth. In each of these methods, images are coded for each raster without regard to the types of the images. Therefore, these methods can be used for most types of images.
Such an image coding method will now be described in detail. Image coding methods can be classified into those for bi-level images and those for multilevel images. The MH, MR, MMR coding methods are those for bi-level images, and are standard coding methods for facsimile machines. Further, recently, a coding method using arithmetic code has been watched, and a coding method called QM-coder is used in a JBIG method which is studied as a standard method in ITU-T (International Telecommunication Union--Telecommunication Standardization Sector) and ISO (International Organization for Standardization).
On the other hand, as the coding methods for multilevel images, there is an ADCT (Adaptive Discrete Cosine Transform) method (called a JPEG method) using DCT (Discrete Cosine Transform) which is studied as a standard method in ITU-T and ISO. In this method, after image information is converted into frequency information, the information is coded.
Accordingly, in an image processing apparatus provided with a coding apparatus in the related art, by using such coding in the related art, an amount of information is compressed, and it is possible to reduce a memory capacity.
In the coding apparatus in the related art, it is possible to efficiently code letter images. However, for images of photographs, images of handwritten letters or the like, it is not possible to sufficiently efficiently code the images.
Further, in a case where letter images are coded, information such as characteristics of the letters are not used, and the letter images are simply treated as images. Therefore, it is not possible to code the images with a very high coding efficiency.